The present invention relates to a silicon wafer to be used as a semiconductor material and having a very low surface defect density with bulk micro defects (BMDs) uniformly and sufficiently abundantly formed therein, and to an epitaxial silicon wafer derived from the silicon wafer by forming an epitaxial layer thereon.
Semiconductor silicon wafers are sliced from silicon single crystals and the method most widely employed for the production of such silicon single crystals is the Czochralski method (CZ method) of pulling single crystals.
The CZ method comprises dipping a seed crystal in a molten silicon placed in a quartz crucible and pulling up the seed crystal to thereby allow a single crystal to grow. With the advancement in the technology of pulling silicon single crystals, it has now become possible to produce less defective, dislocation-free, large single crystals. Semiconductor devices are produced from wafers or substrates prepared from single crystals via a large number of processes. In the course thereof, the substrates are subjected to a large number of physical treatments, chemical treatments and, further, thermal treatments, including treatments in a severe thermal environment, such as high temperature treatments at 1,150xc2x0 C. or above. Thus, problems are produced not only by such defects as oxygen-induced stacking faults (hereinafter referred to as xe2x80x9cOSFsxe2x80x9d), which manifest themselves in device manufacturing processes and lower the performance characteristics of the devices, but also by micro defects, namely grown-in defects, which are formed in the step of crystal growth and greatly affect the performance characteristics of the devices.
FIG. 1 shows the results of an observation of the distribution of typical grown-in defects. This is a schematic representation of the results of an observation of the distribution of micro defects, by X-ray topography, on a wafer sliced from a single crystal just after growing, immersed in an aqueous solution of copper nitrate for deposition of copper and then subjected to heat treatment. On this wafer, OSFs are found distributed in a ring-like manner. Inside the ring, there are detected defects having a size of about 0.1 to 0.2 xcexcm, called laser scattering tomography defects or crystal-originated particles (COPs), for instance, at a density of about 105 to 106 defects/cm3 and, outside the ring, there is a region where there are about 103 to 104 defects/cm3 called dislocation clusters with a size of about 10 xcexcm.
OSFs are stacking faults caused by interstitial atoms and formed on the occasion of thermal oxidation treatment. When formed and grown on the wafer surface, which constitutes active regions of devices, they cause a leakage current and deteriorate the device characteristics. Dislocation clusters, another kind of grown-in defects, too, give no good devices when these are formed thereon. Laser scattering tomography defects act as a factor lowering the time-zero dielectric breakdown characteristic.
Generally, the sites of occurrence of the above defects are greatly influenced by the pulling rate on the occasion of single crystal pulling and by the temperature distribution within the single crystal just after solidification. For example, when a single crystal is grown while gradually lowering the pulling rate and it is examined for the distribution of various defects in a plane cut longitudinally along the pulling axis in the center of the crystal, the results schematically shown in FIG. 2 are obtained. Thus, in the stage of higher pulling rates after shoulder formation and attainment of a desired single crystal diameter, there are ring-forming OSFs in the peripheral portion of the crystal and the inside is a region where a large number of laser scattering tomography defects occur. With the decrease in pulling rate, the diameter of ring-forming OSFs becomes gradually smaller and finally null, whereupon the whole wafer surface becomes a region of occurrence of dislocation cluster defects alone, which corresponds to the region outside the ring-forming OSFs shown in FIG. 1. Thus, FIG. 1 shows the wafer sliced at position A in FIG. 2 or from a single crystal grown at the corresponding pulling rate.
In pulling up single crystals in the art, the pulling rate in single crystal growth has been increased and so controlled that the site of ring-forming OSFs, which is a region allowing high density occurrence of OSFs, may be shifted to the outer periphery of the crystal, since laser scattering tomography defects are not so adversely influential than dislocation clusters, and for the effect of productivity improvement.
However, various investigations have been made to provide methods of producing single crystals from which wafers can be obtained with the number of these defects being reduced as far as possible. Upon more detailed observation of the wafer shown in FIG. 1, there is found an oxygen precipitation promoted region, which is defect-free and in which oxygen precipitation tends to occur, just outside and adjacent to the ring-forming OSFs and, outside that region, there is an oxygen precipitation inhibited region, which is defect-free and in which oxygen precipitation hardly occurs, then followed by a region allowing the occurrence of dislocation cluster defects. There is also a denuded zone inside the ring-forming OSF region between that region and the laser scattering tomography defect region. The state of distribution of these ring-forming OSF region and the neighboring regions varies depending on the temperature distribution within the single crystal just after pulling up and/or the pulling rate and, in these oxygen precipitation promoted region and oxygen precipitation inhibited region, the occurrence of grown-in defects is very infrequent. Therefore, technologies have been developed to enlarge such portions to the whole single crystal to thereby obtain defect-free wafers.
Thus, according to the invention disclosed in Japanese Patent Application Laid-open No. H08-330316, for instance, the temperature gradient within crystal G (xc2x0C./mm) in the pulling axis direction in the temperature range from the melting point to 1,300xc2x0 C. is controlled so that the ratio V/G (where V is the pulling rate (mm/min) in single crystal growth) in the internal portion from the center of the crystal to 30 mm from the periphery may amount to between 0.20 and 0.22 [mm2/(xc2x0C.xc2x7min)] and this ratio may be gradually increased toward the periphery. By carrying out the pulling in that manner, the denuded zone comprising the oxygen precipitation promoted region and oxygen precipitation inhibited region outside the OSF ring alone can be extended to the whole section perpendicular to the pulling axis, hence to the whole single crystal. In this case, it is indicated that the positions of the crucible and heater, the position of the semiconical thermal radiator made of carbon and disposed around the growing single crystal, the structure of the thermal insulator around the heater and other various conditions should be examined by overall heat transfer calculations so that the above temperature conditions may be selected for the crystal growth.
If schematically illustrated in the same manner as in FIG. 2, the case where this method is employed may be illustrated as shown in FIG. 3. That is, when a single crystal grown while gradually decreasing the pulling rate is examined for the distribution of various defects in a section cut longitudinally along the crystal center pulling axis, it is found that the V-shaped region of occurrence of ring-forming OSFs as shown in FIG. 2 is converted to a U-shaped one by changing the temperature distribution within the single crystal just after pulling up. Thus, when a single crystal is grown at a pulling rate indicated by E, the whole crystal is occupied by a denuded zone and defect-free wafers can thus be obtained.
For attaining such a defect-free state, however, the condition ranges are restricted and it is not easy to stably realize the increase in pulling rate and the enlargement in single crystal diameter, for instance, on the place of mass production.
Recent trends toward decreases in temperature in device production steps and toward reductions in high temperature heat treatment time have rendered the effects of OSFs less injurious and made them less meaningful as factors deteriorating device characteristics. On the other hand, as the trend toward miniaturization of circuits increases, laser scattering tomography defects, which are grown-in defects, have become causative of decreases in the yield of conforming products and it is now an important problem to reduce the density of laser scattering tomography defects.
From such a viewpoint, Japanese Patent Application Laid-open No. H11-147786 discloses an invention according to which a silicon crystal is pulled up within the temperature range from the melting point of silicon to 1,400xc2x0 C. in the center of the crystal so that V/G may amount to between 0.112 and 0.142 [mm2/(xc2x0C. xc2x7min)], when explained using the same symbols as used in the above-cited Japanese Patent Application Laid-open No. H08-330316. By doing so, it is intended that the form of the region of occurrence of ring-forming OSFs and the denuded zones on both sides thereof be enlarged to thereby obtain silicon single crystal wafers containing the ring-forming OSF occurrence region but free of the dislocation cluster defect occurrence region or laser scattering tomography defect occurrence region.
When schematically illustrated, the above invention is considered to consist in a method of producing single crystals under conditions close to those indicated by D in FIG. 3. It is alleged that when the occurrence of ring-forming OSFs is prevented by reducing the oxygen content to 24 ppma or below, defect-free wafers can be obtained under a relatively wide range of single crystal production conditions.
It has become known that those defects originating in the process of growing a single crystal and exerting great influences on the characteristics of devices formed on wafers taken from the single crystal can possibly be reduced by growing the single crystal so that the above-mentioned denuded zone intrinsic in the single crystal can be enlarged.
While one of the characteristics required as wafers is that they have a minimized number of defects, as mentioned above, it is also important that they, as device substrates, have a gettering effect on the heavy metal contamination which is unavoidable in the production process. Defects called bulk micro defects (BMDs) which occur in the wafer are effective on this gettering effect, and oxide precipitates formed in the region of occurrence of laser scattering tomography defects are considered one species thereof.
Meanwhile, the denuded zone outside the region of occurrence of ring-forming OSFs on a wafer is divided into two regions, namely the oxygen precipitation promoted region adjacent to the OSF occurrence region and the oxygen precipitation inhibited region outside the promoted region. While, in the oxygen precipitation promoted region, oxide precipitates forming BMDs can be obtained, the formation of BMDs in the oxygen precipitation inhibited region tends to be difficult. In practicing the invention described in the above-cited Japanese Patent Application Laid-open No. H08-330316 which comprises extending the denuded zone outside the ring-forming OSF occurrence region, it is expected that there may occur parts inferior in gettering capability due to the insufficient occurrence of BMDs, since the above-mentioned oxygen precipitation inhibited region is contained in the product. Further, according to the invention disclosed in Japanese Patent Application Laid-open No. H11-147786, both the denuded zones inside and outside the ring-forming OSF occurrence region are to be expanded, so that wafers differing in gettering effect from place to place may possibly be obtained.
The active layer on the wafer surface on which devices are to be formed is required to be as low as possible in defect density. Further, wafers having an active layer with a reduced defect density can also be produced by rapid temperature raising and lowering heat treatment or rapid thermal annealing (RTA) treatment for defect-free layer formation, or by epitaxial layer formation. Even in the case of such layer formation, defect-free better wafers can be obtained when the substrate defect density is as low as possible. On the other hand, it is desirable that, in any of these wafers, a sufficient number of BMDs be formed therewithin to show a good gettering effect.
It is an object of the present invention to provide a wafer having a very low surface defect density with BMDs formed uniformly and sufficiently abundantly as well as a method of producing the same.
The present inventors made various investigations in an attempt to establish a method of producing single crystals from which wafers having a minimized density of grown-in defects, such as dislocation clusters and laser scattering tomography defects, and having a sufficient number of BMDs to show a gettering capability can be obtained.
A typical investigation was carried out as follows. First, single crystal growth was carried out to attain a diameter of 210 mm and a body length of 1,000 mm by continuously decreasing the pulling rate from 1.5 mm/min to 0.3 mm/min. In that case, the temperature distribution within the single crystal just after solidification was controlled so that, in the longitudinal section parallel to the pulling axis, the ring-forming OSF occurrence region might become U-shaped, as schematically shown in FIG. 3, and the area free of either of the dislocation cluster defect occurrence region and the laser scattering tomography defect occurrence region might be enlarged.
When wafers are sliced from such a single crystal perpendicularly to the pulling axis, the wafer sliced at the position C in the figure corresponds to the wafers produced on the mass production sites of the prior art. The position B corresponds to a wafer resulting from a higher pulling rate and a higher cooling rate, the position D to a wafer comprising a denuded zone containing a ring-forming OSF occurrence region, and the position E to a defect-free wafer comprising a denuded zone outside the ring-forming OSF occurrence region. Wafers were sliced at the respective positions B, C, D and E of that single crystal and examined for the occurrence of defects, the formation of BMDs and the time-zero dielectric breakdown (TZDB) characteristic and so on, and further for the conformity to heat treatment for defect-free layer formation or to epitaxial layer formation.
The wafer sliced at the position B, in which the ring-forming OSF occurrence region is found approximately on the periphery, is a wafer uniform from the viewpoint on the defect distribution and containing BMDs formed uniformly in radial directions. However, because of the high density occurrence of laser scattering tomography defects small in size, it has been scarcely possible to obtain wafers satisfactory in time zero dielectric breakdown characteristic.
The wafer at the position D gave good results, namely it was almost free of grown-in defects. However, the formation of BMDs was site-dependent, namely the central portion of the wafer had a larger number of them but the peripheral portion had a smaller number of them. The wafer at the position E is excellent from the viewpoint on the defect-freeness but the drawback thereof is that a sufficiently large number of BMDs cannot be formed.
Based on such examinations, it may be said that the wafers at the position C, which are generally used in ordinary production processes, are suited for practical use from the viewpoint of sufficient BMD formation under the control of oxygen content, of the homogeneity of each wafer as a whole and, further, of productivity and so on. There is the problem, however, that when they are used as substrates for epitaxial wafers, many traces of defects remain upon epitaxial layer formation, hence satisfactory layers are not always obtained.
When the wafers corresponding to the position C were examined using the above-mentioned single crystal showing a U-shaped region of occurrence of ring-forming OSFs, it was found that laser scattering tomography defects become greater and the density thereof decreases as the pulling rate decreases, namely as the position comes close to the OSF ring. As the position further came close to the ring of OSFs and the diameter thereof decreased, the central portion became a denuded zone where almost no defects appeared. It seemed, however, that there be a region where defects are small in size and low in density between the region of occurrence of laser scattering tomography defects and the denuded zone.
On the other hand, in growing single crystals, it is one of objects to easily obtain defect-free single crystals having no grown-in defects, and for enlarging the denuded zone which is adjacent to the ring-forming OSF occurrence region, investigations were also made in order to enlarge the horizontal portion of the U-shaped bottom portion in the U-shaped distribution shown in FIG. 3. As a result, it was found that the U-shaped bottom horizontal portion can be expanded if two conditions are satisfied simultaneously, namely if the temperature gradient in the pulling axis direction in a temperature range just after solidification is increased as far as possible and if the temperature gradient in the pulling axis direction in the central portion of the single crystal is made greater than that in the peripheral surface portion.
Accordingly, single crystal growth was attempted while varying the pulling rate in the same manner as shown in FIG. 3, using the cooled portion, namely the hot zone, of the single crystal where the temperature distribution becomes such that the horizontal portion of the U-shaped bottom is enlarged. An example of that case is schematically shown in FIG. 4. It was thus found that when the single crystal is grown under conditions that the horizontal portion of the U-shaped bottom can be enlarged, a region where defects small in size are distributed at a low density occurs enlarged between the denuded zone above the ring-forming OSF occurrence region and the laser scattering tomography defect occurrence region.
A wafer was sliced from that low-defect region (hereinafter referred to as xe2x80x9cX regionxe2x80x9d) and this wafer (X wafer) was examined in detail. It was found that although the defect size was almost the same or smaller as compared with the above-mentioned wafer at the position B, in FIG. 3, of the single crystal pulled up at a high rate, the defect occurrence density was by far lower. When tested for time-zero dielectric breakdown, the X wafer gave good results as anticipated from the state of defects. Further, upon oxygen precipitation treatment, satisfactory BMDs could be formed. It was thus revealed that an ideal wafer can be obtained in the above manner.
The reason why a region with small defects distributing therein at a low density is formed between the denuded zone and the laser scattering tomography defect occurrence region, as mentioned above, is still unknown. However, for causing the ring-forming OSF occurrence region to have a shape resulting from enlargement of the horizontal portion of the U-shaped bottom, as shown in FIG. 4, it is necessary to increase the temperature gradient in the pulling axis direction just after solidification and to further control the temperature gradient in a manner such that the central portion of the single crystal shows a greater temperature gradient than the peripheral surface portion, and the behaviors of vacancies and interstitial silicon atoms within the single crystal having such temperature distribution conditions just after solidification may presumably contribute to the formation of the region in question.
It is not always easy to increase the temperature gradient in the pulling axis direction and control that temperature gradient in a manner such that the central portion of the single crystal shows a greater gradient than the peripheral surface portion. Since the heat of the single crystal just after solidification is lost mainly from the surface, it becomes necessary to enhance the heat transfer from the single crystal surface so that the temperature gradient in the pulling axis direction may be increased. However, this means that the surface is cooled more intensely and, as a result, the temperature gradient in the surface portion becomes greater than the single crystal inside.
On the contrary, the present inventors made investigations concerning the relative positions of the crucible and heater, the utilization of a heat shield, namely the shape of the heat shield, the distance between the melt surface and heat shield, the distance between the heat shield or cooling member and the single crystal surface and other factors and, as a result, realized a temperature distribution such that the temperature gradient in the central portion is greater than in the surface portion. And, first, a single crystal is grown while varying the pulling rate, and the pulling rate at which the desired region can be obtained is determined. By carrying out growing at that rate, it becomes possible to attain such a defect distribution state over the whole single crystal length.
Wafers sliced from the single crystal obtained in that manner are very small in the number of grown-in defects. When these wafers were subjected to heat treatment for oxide precipitate formation, BMDs could be formed to a satisfactory extent to give less defective wafers excellent in gettering effect. Furthermore, heat treatment for defect-free layer formation could readily result in the formation of a defect-free layer on the surface and, thus, wafers abundant in BMDs and suited for use as semiconductor substrates could be obtained. Moreover, epitaxial layer formation treatment gave a defect-free good epitaxial layer and, after further heat treatment for oxide precipitate formation, epitaxial silicon wafers excellent in gettering effect were obtained.
Based on such findings as mentioned above, the inventors further established the critical conditions for obtaining still better wafers in a stable manner and have now completed the present invention. The present invention consists of:
(1) A silicon wafer sliced from a silicon single crystal pulled up and grown by the CZ method, characterized in that the laser scattering tomography defect occurrence region accounts for at least 80% of the wafer surface area and that the laser scattering tomography defects have a mean size of not more than 0.1 xcexcm, with the density of those defects which exceed 0.1 xcexcm in size being not more than 1xc3x97105 cmxe2x88x923.
(2) A silicon wafer as defined above under (1) which has an oxygen concentration of not less than 25 ppma (OLD ASTM).
(3) A silicon wafer as defined above under (1) or (2) which has an oxide precipitate density, as measured on a section, of not less than 1xc3x97104/cm2 as resulting from heat treatment for oxide precipitate formation.
(4) A silicon wafer as defined above under (1) or (2) which has a defect-free layer on the surface as resulting from heat treatment for defect-free layer formation.
(5) An epitaxial silicon wafer which comprises an epitaxial layer formed on the surface of a silicon wafer as defined above under (1), (2), (3) or (4).
(6) An epitaxial silicon wafer which comprises an epitaxial layer formed on the surface of a silicon wafer as defined above under (1) or (2) and has an oxide precipitate density, as measured on a section, of not less than 1xc3x97104/cm2 as resulting from heat treatment for oxide precipitate formation within the wafer following epitaxial layer formation.
(7) An epitaxial silicon wafer which comprises an epitaxial layer formed on the surface of a silicon wafer as defined above under (4) and has an oxide precipitate density, as measured on a section, of not less than 1xc3x97104/cm2 as resulting from heat treatment for oxide precipitate formation within the wafer following epitaxial layer formation.
(8) A silicon wafer as defined above under (1) or (2) as sliced from a silicon single crystal produced by pulling by the CZ method under pulling conditions such that while the temperature of the central portion of the single crystal being pulled up from the melt is within the range from the melting point to 1,370xc2x0 C., the temperature gradient Gc in the central portion in the single crystal pulling axis direction is not less than 2.8xc2x0 C./mm and the ratio Gc/Ge, where Ge is the temperature gradient in the peripheral portion in the pulling axis direction, is not less than 1.